Why can't he send troops into Mexico? Hello? ILLEGALS are invading our country! An invasion is actually a declaration of war.
As for the birthright citizenship, yes that would be difficult, but not impossible. Certainly it would be challenged and eventually head to SCOTUS where they would have to rule.
Illegal immigrants entering. The country is not an act of war, but an illegal act. We have laws already on the books that deal with this issue. They’re just not being enforced.
And if you want to change birthright citizenship, you have to change the constitution. I can tell you the votes are not there.